PHOTOGRAPHY, the art of produc ing permanent pictures by means of the chemical action of light on sensitized sur faces. The first to produce permanent pictures by the chemical influence of light was M. Niepce at Chalais-sur-Mer, in 1814, and his idea of the development of a latent image by treatment after exposure lies at the basis of photography. Niepce and Daguerre discovered that they were conducting experiments of a kindred character, and in 1829 entered into partnership. Daguerre, with Niep ce's son Isidore, carried on this work in 1839, and invented the process known as the daguerreotype.
The introduction of collodion in 1857 was a great step in photography. The collodion process is divided into five stages: (1) The preparation of a clean glass plate by coating with collodion to which bromide of cadmium and either iodide of potassium or iodide of ammo nium has been added. (2) The sensitiz ing of the collodion by immersion in a bath of nitrate of silver, and distilled water. (3) The production of a latent image by exposing the sensitized plate in the camera. (4) The development of latent into visible image by immersing the plate in a solution of sulphate of iron or pyrogallic acid, to either of which some acetic or citric acid is added. (5) Fixing of the permanent image by im mersion of the plate in some solvent of those parts of the sensitive surface upon which the light has acted. This solvent for wet plates is cyanide of potassium, but for more modern processes hyposul phite of sodium is employed. On account of the trouble of preparing the wet col lodion plates, the dry plates, made by the gelatine-bromide process., have almost universally taken their place. Dry plates made by some form of this gelatine bromide process are manufactured on a large scale. When properly made they are much more sensitive than the col lodion plates and will keep good for years, and can be developed months after exposure. The positive prints are made by the action of light, passing through the developed plate, on a sensitized pa per, over which the plate is placed. The silver, platinum, and ferro-prussiate pa pers are the most used.
Silver Process.—Suitable paper is coat ed on one side by a thin layer of albu men, to which chloride of sodium or of ammonium has been added. The old method of dispensing with albumen and using paper which is salted only, is re viving. The paper is sensitized by float ing it on a solution of silver nitrate, and drying. After printing, the paper is treated with a solution of chloride of gold, which tones it, producing different shades of color, depending on the length of immersion and strength of solution.
After toning, the print is fixed in the same manner as the plate, by a solution of hyposulphite of sodium, which removes the undarkened silver salts. Gelatine or collodion is sometimes used in place of albumen in this silver-printing process. All soluble substances have to be washed from the prints after they have been toned and fixed because otherwise the photographs become ultimately discol ored. The papers known as aristotype, argentotype, and celerotype are gelatine emulsions of chloride of silver spread on paper.
Platinum Process.—An image can be obtained in platinum black if the paper is sensitized with ferric-oxalate with which is mixed a solution of chloro-pla tinite of potassium. The action of light on this paper reduces the ferric salt to the ferrous state, and when the ferrous salt is in solution the platinous salt is reduced by it. By floating the exposed paper on a solution of neutral potassium oxalate, which is a solvent of the ferrous oxalate, the platinum salt in contact with it is immediately reduced to the metallic state, and an image is thus built up. To fix the prints they are immersed in dilute hydrochloric acid, which dissolves away the ferric oxalate, and the oxalate of lime.
Ferroprussiate Process.—The ordinary method of making blue prints.
Composite photography, a method of superposing several or many photo graphs, thus getting a sort of average of the whole and showing the type. If it is a human composite photograph type, then the eyes of each sitter are brought to the same place on the lens, and the exposure for each is very short.
The most important advance in pho tography in recent years has been made in the direction of taking photographs in natural colors. Many scientists have worked on the problem and several proc esses have been developed. The most remarkable of these was perfected in 1907 by Antoine Lumiere and his sons August and Louis, of Paris, who suc ceeded in taking color photographs on a single plate and in an ordinary camera, with exposures of one second and less. Their process consists in the formation of a color screen on a glass plate by plac ing on it a layer of microscopic grains of transparent potato starch, in three por tions colored respectively orange, green, and violet; the plate is then sensitized by coating with a gelatin-bromid emul sion. After the exposure, the plate is developed by a double process that turns it to a positive and the result is a beau tiful transparency in the natural colors.